The permanent magnet motor which is one of the motors has been conventionally well known. In regard to the permanent magnet motor, one whose stator field core is provided with a supplemental groove for reducing a cogging torque is disclosed in the document of Japanese Patent Publication No. SHO 58-42708, and a further one whose stator field core is provided with a supplemental salient pole and a supplemental groove is disclosed in the document of Japanese Patent Publication No. HEI 6-81463 and so on.
FIG. 23A is a view showing an example of the construction of a prior art permanent magnet motor provided with a supplemental groove for reducing the cogging torque, while FIG. 24 is an enlarged view showing the essential part of the motor shown in FIG. 23A.
As shown in FIG. 23A, this prior art permanent magnet motor is substantially constructed of a stator field core 1 and a rotor 2. In this case, six permanent magnetic poles 3 are fixed on the periphery of the rotor 2. That is, the rotor 2 is the permanent magnet rotor. Further, a plurality of supplemental grooves 5 are formed at regular intervals in a rotor circumference direction at a salient pole section 1a of the stator field core 1. It is to be noted that a winding slot 7 is provided between adjacent stator field cores 1 (salient pole sections 1a).
As shown in FIG. 24, in the above permanent magnet motor, assuming that P is an integer not smaller than one, then the number of the permanent magnetic poles 3 (magnetic pole count) fixed to the rotor 2 is generally set to 2P, and the number of the salient pole sections 1a (salient pole count) of the stator field core 1 is set to 3P. Further assuming that an angle corresponding to the length of the stator field core 1 in the rotor circumference direction is 3.theta., then two supplemental grooves 5 of the salient pole section 1a of the stator field core 1 are arranged so that an angle corresponding to the interval in the rotor circumference direction is .theta.. In this case, an angle corresponding to the length in the rotor circumference direction of each permanent magnetic pole 3 is 4.5.theta.. In this case, when the supplemental grooves 5 are not provided, the degree of the lowest common multiple of the number of the permanent magnetic poles 3 (magnetic pole count) and the number of the salient pole sections 1a (salient pole count) of the stator field core 1 is the cogging torque per rotation of the rotor 2, and the cogging torque is 6P (the lowest common multiple of 2P and 3P) per rotation in this case.
In contrast to this, when the supplemental grooves 5 are provided, the salient pole sections are apparently increased in number, and the cogging torque is 18P (the lowest common multiple of 2P and 3.times.3P) per rotation in this case. However, in this case, actually a triple higher harmonic (the degree of 18P per rotation) is superimposed on a fundamental wave (the degree of 6P per rotation).
Specifically, in a case where P=3, the fundamental wave of the cogging torque has 18 waves per rotation (one wave per 20.degree.). However, a waveform including the triple higher harmonic on its fundamental wave results since the supplemental grooves 5 are provided as shown in FIG. 23B, when a cogging torque waveform 6b does not become a cogging torque having only the higher harmonic component of 54 waves per rotation.
As described above, the apparent cogging torque is reduced by incorporating the high-degree components into the fundamental wave component of the cogging torque with the provision of the supplemental grooves at regular intervals in the prior art stator field core, however, it does not have the optimum supplemental groove arrangement capable of sufficiently removing the fundamental wave component.
Furthermore, in the prior art permanent magnet motor in which the stator field core is provided with the supplemental salient pole section and the supplemental groove, there is the problem that the winding is hard to be achieved since the supplemental salient pole section is formed on the stator field core. For the purpose of improving this problem, there can be considered a measure for dividing the stator field core into a plurality of cut type cores. However, in this case, the cores are increased in number to incur an increased number of assembly processes and cause a problem that the structural strength of the stator field core is weakened.
Furthermore, the prior art permanent magnet motor has the problem that the torque pulsation increases depending on the control system even though its cogging torque is low.
The present invention has been developed to solve the aforementioned conventional problems, and its object is to provide a permanent magnet motor having a low cogging torque or a permanent magnet motor having a reduced torque pulsation capable of arranging supplemental grooves in optimum positions and being assembled through the assembly processes equivalent in number to those of the prior arts while assuring the structural strength equivalent to those of the prior arts even when the stator field core is divided into cut type cores.